Conventionally, a thermally expandable sheet (or thermally foamable sheet), in which a thermally expanding layer (or foaming layer) incorporating foamable microcapsules that is expanded by heating is formed on one face side of a substrate sheet, has been known. An image pattern having high optical absorptivity is printed on this thermally expandable sheet, followed by irradiation of light containing infrared radiation, and then a thermally expanding layer of an area corresponding to the image pattern is selectively heated and expanded, whereby a three-dimensional image corresponding to the image pattern is able to be formed on one face side of the substrate sheet.
With respect to such a three-dimensional image forming technique, for example, Unexamined Japanese Patent Application Kokai Publication No. S64-28660 describes a method for forming a three-dimensional image, in which a print image is formed on a surface of a thermally expanding layer side of a thermally expandable sheet or a back face of a substrate sheet side thereof using a black toner or ink having high optical absorptivity, and then the print image is caused to absorb light by light irradiation using a halogen lamp or the like to generate heat, whereby microcapsules of the thermally expanding layer in an area corresponding to the print image are heated and expanded (or foamed).
Further, for example, Unexamined Japanese Patent Application Kokai Publication No. 2001-150812 describes a method for adjusting a raised height of a three-dimensional image, in which a color image or the like is formed on a surface of a thermally expanding layer side of a thermally expandable sheet, and then a light absorption pattern including gray-scale images corresponding to pictures and others of the color image of the surface is formed on a back face of a substrate sheet side, followed by light irradiation from a back face side of the thermally expandable sheet to generate heat corresponding to shades of the light absorption pattern, whereby an expanded amount of the thermally expanding layer is controlled.
Incidentally, there is also proposed a method in which a three-dimensional image is initially raised on a thermally expandable sheet and then, for example, an ink of white which is a background color is coated on the entire face of a thermally expanding layer side or without such coating, a color image is formed thereon.
The method described in the aforementioned Unexamined Japanese Patent Application Kokai Publication No. 2001-150812 is considered to be able to form a three-dimensional image with any portions controlled to have any appropriate raised heights (foamed heights) in response to pictures of a color image and the like formed on a surface of a thermally expanding layer side of a thermally expandable sheet.
However, the present inventors have examined such methods for forming a three-dimensional image, and as a result, found a problem in which upon forming a three-dimensional image on a thermally expandable sheet by light irradiation using a long halogen lamp or the like, an expanded (foamed) height when a thermally expanding layer is expanded and raised differs from one to be expected, resulting in non-uniformity. This is a defect caused by a fact that a light/heat irradiation temperature of a longitudinal direction end portion of the halogen lamp for light/heat irradiation on the thermally expandable sheet is lower than in a central portion.
In general, a long halogen lamp is formed by enclosing a halogen heater power-supplied from its both end terminals in a glass tube. The distribution of its light/heat irradiation temperature indicates a constant temperature distribution in a certain distance from the central portion toward the end portion. However, naturally, the temperature markedly decreases in a vicinity of the end portion. Therefore, commonly, the halogen lamp is used in a constant temperature distribution range excluding both end portions.
However, such a manner causes a size of a three-dimensional image forming apparatus to depend on a dimension of the halogen lamp. Therefore, a problem to be solved that a size of the apparatus increases has been produced. Problems in the prior art will also be described in detail in an embodiment to carry out the invention to be described later.